1. Field of the Invention
The present invention relates to a testing device for checking the quality of a paint or lacquer layer, and a corresponding method.
2. Description of the Related Art
In latex paints and lacquers, solvents or water as well as other volatile substances escape from the still wet layer after application. Since the composition of the lacquer is therefore constantly changing during the drying phase, flocculation (reagglomeration) of the pigments contained therein may occur. In addition, demixing of pigments having different densities, for example titanium dioxide and copper phthalocyanin, may occur. In the worst-case scenario, hexagonal cells, so-called Bénard cells, are formed which are caused by flow within the lacquer layer.
Shear forces are introduced into the system as the result of rubbing during drying. It is thus possible to disaggregate (deagglomerate) the agglomerates, i.e., eliminate the demixing. The rubbed surface may thus have an appearance which is significantly different from, usually more intensely colored than, the unrubbed surface.
The difference between a rubbed surface of a paint or lacquer layer and an unrubbed reference surface of the paint or lacquer layer is referred to as the “rubout effect.” The rubout effect is stronger the more intensely colored/weakly colored is the appearance of the rubbed surface in comparison to the unrubbed surface. If the rubbed surface has a more intensely colored appearance than the unrubbed surface, this is referred to as a positive rubout effect, and if the rubbed surface has a more weakly colored appearance, this is referred to as a negative rubout effect. In the ideal case, no differences in color intensity or hue are apparent.
The rubout test is conventionally carried out manually, in that the person conducting the test rubs a bare finger, a gloved finger, or a brush over the test region to be checked. Depending on the paint or lacquer system, rubbing with the bare finger may have adverse health effects over the long term. On the other hand, the viscosity limit is practically imperceptible using laboratory gloves. When a brush is used for testing, the brush must be laboriously cleaned, which is one reason, among others, that this method is seldom used.
In addition, defining a test period is problematic. For example, a rubout test using the bare finger may be carried out until the lacquer “pulls up,” i.e., until a specific limiting viscosity of the lacquer which is perceived by the tester is reached. However, identifying an end point independently of the tester is extremely difficult. On the other hand, setting a defined test period, for example in order to standardize the overall energy input during the testing, does not take into account differences between testers or different drying times due to system-related factors, since breakup of the flocculate is effective only when it is also stabilized by the advanced-stage drying.
Lastly, the conditions under which a rubout test is to be carried out have not been normatively defined. In most cases, the result of the rubout test is highly dependent on the tester and his physical and mental state on a given day. In addition, the results from different laboratories are rarely comparable due to differing procedures. Approaches according to which checking of the quality of a paint or lacquer layer may be automated are described in U.S. Pat. No. 2,316,518 A1, U.S. Pat. No. 2,287,148 A, U.S. Pat. No. 3,040,559 A, and U.S. Pat. No. 3,974,678.